\begin{tikzpicture}
    \pgfmathsetmacro{\R}{2}
    \pgfmathsetmacro{\n}{6}

    \tkzDefPoints{0/0/O}
    \tkzDefPoint(240:\R){A} % 270 - 360/6/2 = 270 - 30 = 240
    \tkzDefRegPolygon[center,sides=\n,name=P](O,A)
    \foreach \P [count=\i from 2] in {B,C,...,F} {
        \coordinate (\P) at (P\i);
    }
    \tkzDefLine[altitude](A,O,B)  \tkzGetPoint{G}

    \tkzDrawCircle[very thick](O,A)
    \tkzDrawPolygon(P1,P...,P\n)
    \tkzDrawSegments[dim={$a_6$,-1.2em,}](A,B)
    \tkzDrawSegments(O,A  O,B  O,G)

    \tkzMarkRightAngle[size=.2](B,G,O)
    \tkzLabelSegment[right](O,B){$R$}
    \tkzLabelSegment[pos=.3, xshift=-.5em](O,G){$r_6$}
    \tkzLabelPoints[above](O)
    \tkzLabelPoints[left,  yshift=-.3em](A)
    \tkzLabelPoints[right, yshift=-.3em](B)
    \tkzAutoLabelPoints[center=O, centered, dist= .2](C,...,F)
    \tkzLabelPoints[above, xshift=-.5em](G)
\end{tikzpicture}

